The present invention relates to laser printers, wherein the laser power is modulated, particularly on and off, for activating or deactivating a photosensitive material in an electro-photographic process, particularly for cut paper.
A semiconductor laser, particularly a laser diode (LD) has recently become the leading laser light source for laser printers. In order to carry out a stable printing operation, the laser output must be kept constant. However, the semiconductor laser has a characteristic of varying its output with temperature. In order to eliminate such an inconvenient power variation with temperature, in an optical disc and an laser printer, a monitor with a photodetector is used to detect the laser output for negative feedback control operation in an attempt to maintain a constant output for the laser. For example, reference is made to Japanese Patent Laid-Open No. 13343/1979.
In these conventional printer techniques, a sample and hold operation is carried out, in which a monitor discretely detects the laser output in response to the laser beam moving past a photodetector provided at the start point of scanning. A negative feedback control operation is performed with respect to this detection so that the output of the laser is maintained constant during the printing time.
It is also known, with respect to the device such as an optical disc device, to have the laser device emit light in two directions, with one direction being used for scanning and the opposite direction being used for detection, which is convenient when the laser is always on such as in the optical disc reading mode.